Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. In addition, the efficiency of conversion of electricity to light has now reached the same levels as obtained in fluorescent light fixtures.
LEDs have limited power, and hence a replacement requires a large number of LEDs. If the conventional light source was a point source or a compact source such as a conventional incandescent bulb, a luminaire manufacturer can start from a printed circuit board on which a plurality of dies have already been mounted together with a power supply that converts AC to a constant current source that drives the LEDs. This prefabricated component can be used in a large variety of luminaires that require a source of the same light output and color temperature. In addition, light sources that provide different light outputs that are less than some predetermined maximum light output can be constructed from the same printed circuit board by partially populating the printed circuit board with LEDs.
However, many applications require a distributed light source. Light fixtures for illuminating work areas in commercial office and retail spaces are typically two-dimensional extended sources. The goal of such light sources is to provide a predetermined level of light flux on a work surface under the light fixture while minimizing the brightness of the source such that a user can look directly at the light source without the glare causing discomfort.
In existing lighting systems, this type of extended light source is typically constructed from a plurality of fluorescent tubes in an enclosure that is covered by some form of diffuser. The number and types of tubes vary from light source to light source. Furthermore, for any given tube form factor, there are variety of tubes depending on the desired output light spectrum or color temperature.
Providing LED replacements for such light sources presents a number of challenges. Upgrading existing light sources typically involves replacing the fluorescent tubes with a LED-based source that has the same form factor as the existing fluorescent tubes. The fluorescent tube form factor limits the intensity of the replacement fluorescent tube because of the difficulties in providing sufficient heat dissipation within the form factor of an existing fluorescent tube. Furthermore, multiple LED-based tubes are required for each light fixture to provide a one-for-one replacement with the existing fluorescent tubes. As a result the cost of upgrading an existing light source is still many times the cost of the fluorescent tubes being replaced.
If a single extended LED-based two-dimensional source is used, a large area printed circuit board is needed having an area similar to that of the existing fixture. The fixture requires in excess of 100 dies to be mounted on a large area that requires expensive equipment to pick and place the dies over the area. In addition, each different luminaire requires a different printed circuit board and associated specialized equipment. In addition, the circuit board must provide the heat-dissipation surface, since LEDs are more sensitive to operating at elevated temperatures.
The number of potential linear and two-dimensional source sizes and configurations is potentially very large. Hence, inventorying all the potential sizes poses significant challenges. In principle, some smaller-sized extended light sources that include a plurality of LEDs and that can be used to construct larger light sources could be stocked; however, such an arrangement requires the luminaire manufacturer to provide a substrate for combining a number of such sources and to perform the fabrication work needed to mount the individual light sources on that substrate. This increases the cost and design cycle time to the luminaire manufacturer.